The liver performs diverse and important metabolic functions in human metabolism; thus, liver dysfunction can lead to serious health consequences. As a result, many different tests are used to assess liver function, but each test has limitations that compromise its value. Since galactose is primarily metabolized by the liver, the galactose breath test represents a noninvasive method of assessing liver function. In this test, 13C- galactose and a carrier dose of unlabeled galactose is administered to the patient and the appearance of 13CO2 is used as an indication of liver function. Although the literature suggests that this test is a good predictor of liver function, the clinical usefulness of the galactose breath test has not been fully evaluated due to the unavailability of 13 C galactose. The goal of this research is to produce 13C galactose and evaluate the clinical usefulness of this compound as a test of liver diseases. 13C galactose will be produced from a microalgae cultured in the presence of 13CO2. The purified 13C galactose will be used in breath tests to optimize the carrier dose of unlabeled galactose, minimize the dose of 13C galactose and determine the sensitivity and specificity of this test for assessing liver function.